Metabolism and Translocation of Exogenous Zeatin Riboside in Germinating Seeds and Seedlings of Zea mays

High specific activity [³H]-zeatin riboside (ZR) was suppliedto germinating seed and developing seedlings of Zea mays tostudy its metabolism and translocation The major metabolitesof ZR in endosperm, embryo, and first leaves were adenosine,adenine, and adenine nucleotide When ZR was supplied to theradicle tip a significant proportion of the radioactivity extractedfrom the radicle was identified as zeatin-9-glucoside (Z9G).However, some ZR was also transported to the shoot and vestigialembryo During the initial stage of germination, movement ofzeatin riboside from the embryo to the endosperm was pronouncedbut little movement occurred in the reverse direction Key words: Zea mays cytokinin, zeatin riboside, metabolism, translocation     

The Metabolism of Zeatin and Zeatin Riboside by Soya Bean Callus

Five cell division inducing compounds were found in soya beancallus irrespective of whether it wes grown on a zeatin or zeatinriboside containing basal medium. In both cases the major metaboliteseems to be zeatin glucoside. The significance of this metabolicstep in plant tissue is discussed.     

Translocation and Metabolism of Injected Maleic Hydrazide in Silver Maple and American Sycamore Seedlings

Studies were conducted to determine the fate of ¹⁴C-maleic hydrazide injected into the stem xylem of 1-year-old silver maple (Acer saccharinum L.) and American sycamore (Platanus occidentalis L.) seedlings. Maleic hydrazide was found to translocate to all parts of the plant within 1 day after treatment. The autoradiographs indicated that the radioactivity was accumulated in meristematic regions of the leaves. A significant amount (over 15% of the applied ¹⁴C) was exuded out of the roots into the nutrient solution after 30 days. The ¹⁴C in the nutrient solution was in the form of unchanged maleic hydrazide, whereas in plant tissue a metabolite possibly a conjugate with sugar was formed. With the passage of time, the amount of metabolite seemed to increase in proportion to that of maleic hydrazide. Approximately 30% of the applied ¹⁴C was unextractable with methanol after 30 days.     

Metabolism and Translocation of Gibberellins in the Seedlings of Pharbitis nil (II). Photoperiodic Effects on Metabolism and Translocation of Gibberellins Applied to Cotyledons

We investigated the metabolism and translocation of two gibberellins(GAs), [³H]GA₂₀ and [³H]GA₁, which were applied at low concentrationto the cotyledons of Pharbitis nil (cv. Violet). Seedlings weregrown under three different photoperiodic conditions: continuouslight (CL-CL), continous light followed by short day conditions(CL-DT) and long day conditions followed by short day conditions(DT-DT). Translocation of the applied [³H]GAs from cotyledonsto hypocotyls was promoted by DT for all GAs examined. Whilethe conversion of the translocated [³H]GA₁ to [³H]GA₈ and itsconjugates was rapid in hypocotyl, the conversion of translocated[³H]GA₂₀ to [³H]GA₂₉ was slow. Radioactivity in epicotyls wasdetected much more rapidly on application of [³H]GA₂₀ than of[³H]GA₁, [³H]GA₈ and [³H]GA₂₉ and their conjugates. The conversionof [³H]GA₂₀ to [³H]GA₁ in the epicotyl was more rapid underCL-CL conditions. This result in consistent with the higherlevel of endogenous GA₁ existing in epicotyls under CL-DT thanDT-DT conditions. However, when [³H]GA₁ was applied to the cotyledon,only small amounts of [³H]GA₈ and its conjugates were detectedin the epicotyl regardless of the photoperiodic conditions.This result may suggest that the translocation and metabolismof [³H]GA₂₀ from cotyledons to epicotyl was faster under CL-CLthan DT-DT conditions and may correlate with the increased epicotylelongation of GA₂₀ treated plants under CL-DT than DT-DT conditions. (Received June 28, 1995; Accepted November 2, 1995)     

Characterization of Zinc Uptake, Binding, and Translocation in Intact Seedlings of Bread and Durum Wheat Cultivars

Durum wheat (Triticum turgidum L. var durum) cultivars exhibit lower Zn efficiency than comparable bread wheat (Triticum aestivum L.) cultivars. To understand the physiological mechanism(s) that confers Zn efficiency, this study used ⁶⁵Zn to investigate ionic Zn²⁺ root uptake, binding, and translocation to shoots in seedlings of bread and durum wheat cultivars. Time-dependent Zn²⁺ accumulation during 90 min was greater in roots of the bread wheat cultivar. Zn²⁺ cell wall binding was not different in the two cultivars. In each cultivar, concentration-dependent Zn²⁺ influx was characterized by a smooth, saturating curve, suggesting a carrier-mediated uptake system. At very low solution Zn²⁺ activities, Zn²⁺ uptake rates were higher in the bread wheat cultivar. As a result, the Michaelis constant for Zn²⁺ uptake was lower in the bread wheat cultivar (2.3 μm) than in the durum wheat cultivar (3.9 μm). Low temperature decreased the rate of Zn²⁺ influx, suggesting that metabolism plays a role in Zn²⁺ uptake. Ca inhibited Zn²⁺ uptake equally in both cultivars. Translocation of Zn to shoots was greater in the bread wheat cultivar, reflecting the higher root uptake rates. The study suggests that lower root Zn²⁺ uptake rates may contribute to reduced Zn efficiency in durum wheat varieties under Zn-limiting conditions.Soils that contain insufficient levels of the essential plant micronutrient Zn are common throughout the world. As a result, Zn deficiency is a widespread problem in crop plants, especially cereals (Graham et al., 1992). The importance of plant foods as sources of Zn, particularly in the marginal diets of developing countries, is well established (Welch, 1993). The development of crop plants that are efficient Zn accumulators is therefore a potentially important endeavor. In addition to its effects on nutrition, Zn deficiency in crops is relevant to other areas of human health. Another consequence of Zn-deficient soils is the tendency for plants grown in such soils to accumulate heavy metals. For example, in the Great Plains region of North America, where soil Zn levels are low and naturally occurring Cd is present, durum wheat (Triticum turgidum L. var durum) grains accumulate Cd to relatively high concentrations (Wolnik et al., 1983). The presence of Cd in food represents a potential human health hazard and, in response, international trade standards have been proposed to limit the levels of Cd in exported grain (Codex Alimentarius Commission, 1993). Thus, there is a need to understand the physiological processes that control acquisition of Zn from soil solution by roots and mobilization of Zn within plants.It has been demonstrated in recent years that crop plants vary in their ability to take up Zn, particularly when its availability to roots is limited. Zn efficiency, defined as the ability of a plant to grow and yield well in Zn-deficient soils, varies among wheat cultivars (Graham and Rengel, 1993). In field trials, durum wheat cultivars have been shown to be consistently less Zn efficient than bread wheat (Triticum aestivum L.) cultivars (Graham et al., 1992). Similarly, durum wheat varieties were reported to be less Zn efficient than bread wheat varieties when grown in chelate-buffered hydroponic nutrient culture (Rengel and Graham, 1995a).The physiological mechanism(s) that confers Zn efficiency has not been identified. Processes that could influence the ability of a plant to tolerate limited amounts of available Zn include higher root uptake, more efficient utilization of Zn, and enhanced Zn translocation within the plant. Cakmak et al. (1994) showed that a Zn-inefficient durum wheat cultivar exhibited Zn-deficiency symptoms earlier and more intensely than a Zn-efficient bread wheat cultivar even though the Zn tissue concentrations were similar in both lines, suggesting differential utilization of Zn in the two cultivars. Rates of Zn translocation to shoots were shown to vary among sorghum cultivars, although correlations with Zn efficiency were not established (Ramani and Kannan, 1985). Root uptake kinetics have been reported to vary between rice cultivars having different Zn requirements, with high-Zn-requiring cultivars exhibiting consistently higher root uptake rates (Bowen, 1986). In contrast, a correlation between Zn efficiency and rates of root Zn uptake in bread and durum wheat cultivars could not be demonstrated (Rengel and Graham, 1995b).In grasses Zn influx into the root symplasm has been hypothesized to occur as the free Zn²⁺ ion (Halvorson and Lindsay, 1977), as well as in the form of Zn complexes with nonprotein amino acids known as phytosiderophores (Tagaki et al., 1984) or phytometallophores (Welch, 1993). Concentration-dependent uptake of free Zn²⁺ ions has been shown to be saturable in several species, including maize (Mullins and Sommers, 1986), barley (Veltrup, 1978), and wheat (Chaudhry and Loneragan, 1972), suggesting that ionic uptake in grasses occurs via a carrier-mediated system. However, several of these studies have been criticized on the basis that excessively high (and physiologically unrealistic) Zn²⁺ concentrations were used (Kochian, 1993).This study was undertaken to examine unidirectional Zn²⁺ influx and translocation to shoots in Zn-efficient bread wheat lines and Zn-inefficient durum wheat lines. Experiments were performed in the absence of added phytometallophores and results are presumed to represent influx of ionic Zn²⁺. Zn activities in the nanomolar range were used to more closely mimic free Zn²⁺ levels occurring naturally in soil solution. The results presented here indicate that a Zn-efficient bread wheat cultivar maintained higher rates of Zn uptake than a Zn-inefficient durum wheat cultivar, particularly at low (and physiologically relevant) solution Zn²⁺ activities.     

Enrichment and Analysis of Intact Phosphoproteins in Arabidopsis Seedlings

Protein phosphorylation regulates diverse cellular functions and plays a key role in the early development of plants. To complement and expand upon previous investigations of protein phosphorylation in Arabidopsis seedlings we used an alternative approach that combines protein extraction under non-denaturing conditions with immobilized metal-ion affinity chromatography (IMAC) enrichment of intact phosphoproteins in Rubisco-depleted extracts, followed by identification using two-dimensional gel electrophoresis (2-DE) and liquid chromatography-tandem mass spectrometry (LC-MS/MS). In-gel trypsin digestion and analysis of selected gel spots identified 144 phosphorylated peptides and residues, of which only18 phosphopeptides and 8 phosphosites were found in the PhosPhAt 4.0 and P³DB Arabidopsis thaliana phosphorylation site databases. More than half of the 82 identified phosphoproteins were involved in carbohydrate metabolism, photosynthesis/respiration or oxidative stress response mechanisms. Enrichment of intact phosphoproteins prior to 2-DE and LC-MS/MS appears to enhance detection of phosphorylated threonine and tyrosine residues compared with methods that utilize peptide-level enrichment, suggesting that the two approaches are somewhat complementary in terms of phosphorylation site coverage. Comparing results for young seedlings with those obtained previously for mature Arabidopsis leaves identified five proteins that are differentially phosphorylated in these tissues, demonstrating the potential of this technique for investigating the dynamics of protein phosphorylation during plant development.     

Metabolism and Long-distance Translocation of Cytokinins

During plant development, distantly-located organs must communicate in order to adapt morphological and physiological features in response to environmental inputs. Among the recognized signaling molecules, a class of phytohormones known as the cytokinins functions as both local and long-distance regulatory signals for the coordination of plant development. This cytokinin-dependent communication system consists of orchestrated regulation of the metabolism, translocation, and signal transduction of this phyto...     

Metabolism and Long-distance Translocation of Cytokinins

During plant development, distantly-located organs must communicate in order to adapt morphological and physiological features in response to environmental inputs. Among the recognized signaling molecules, a class of phytohormones known as the cytokinins functions as both local and long-distance regulatory signals for the coordination of plant development. This cytokinin-dependent communication system consists of orchestrated regulation of the metabolism, translocation, and signal transduction of this phytohormone class. Here, to gain insight into this elaborate signaling system, we summarize current models of biosynthesis, trans-membrane transport, and long-distance translocation of cytokinins in higher plants.     

Translocation and Metabolism of Endosperm-Applied [2-C] Indoleacetic Acid in Etiolated Avena sativa L. Seedlings

The role of free indole-3-acetic acid (IAA) in the endosperm of Avena sativa L. seedlings was investigated to determine its contribution to free IAA in the shoot. [2-¹⁴C]IAA was injected into the endosperm of darkgrown seedlings and the transport and metabolism of the [¹⁴C]-labeled compounds determined. It was concluded that translocation of free IAA directly from the endosperm is probably not a significant source of free IAA in the shoot, mainly because even small amounts of [¹⁴C]IAA introduced into the endosperm were rapidly metabolized. This suggested that, in Avena, free IAA does not normally exist in the liquid endosperm.     

The Metabolism of Ethanol in Germinating Pea Seedlings

Considerable losses of ethanol occurred during the germinationof green pea seeds which could not be ascribed to losses dueto the volatility of the alcohol. Changes in the contents ofacetaldehyde, acetone, organic acids, and in the gas exchangessuggested that the alcohol was oxidatively metabolized. Feedingethanol to slices of pea cotyledon tissue also indicated ethanolconversion to acetaldehyde and the interconversion of acetaldehydeand acetone. Feeding ethanol 2:¹⁴C to the slices confirmed thatthe ethanol was metabolized, giving similar changes in the contentof carbonyl compounds and organic acids to those observed inthe intact germinating pea seedlings. Thus the endogenous ethanolwhich accumulated when pea seed imbibed water prior to germinationmay be metabolized subsequently by the germinating seedling.     

Structural Changes in Radish Seedlings Exposed to Cadmium

Radish (Raphanus sativus L. cv. Redondo Vermelho) seedlings were analysed by light and scanning electron microscopy to characterize the structural changes caused by the exposure to 0.5 or 1.0 mM cadmium chloride for 24, 48 and 72 h. The analyses showed changes in the anatomical and morphological characteristics of roots, stems and leaves of two-week-old seedlings. In all tissues, pressure potential was decreased. Premature death with the disintegration of the epidermis and an increase in the number of root hairs was observed in roots exposed to Cd. The stem of seedlings exposed to Cd exhibited more cells layers in the cambial region. The main effects observed in leaves in response to Cd were stomatal closure, lack of cell wall thickening and alterations in the shape of the chloroplasts. It is suggested that the structural changes observed in seedlings treated with Cd were mainly caused by a Cd-induced decrease in water uptake.     

The Metabolism of Hormones during Seed Germination and Release from Dormancy: III. The Effects and Metabolism of Zeatin in Dormant and Nondormant Ash Embryos

Zeatin and zeatin-9, β-ribonucleoside enhance the germination of dormant ash embryos. While the first macroscopic signs of germination appear only after about 72 hours, 12 hours of exposure to 50 μm zeatin is as effective as continuous incubation. There must be barriers against transport out of the embryos since 8-¹⁴C-zeatin and its metabolites, zeatin-9, β-ribonucleoside, the 5′-mono and the suspected di- and triphosphates, accumulate against a concentration gradient. Zeatin ribonucleoside is about as effective as zeatin in enhancing embryo germination, yet the internal 8-¹⁴C-zeatin level is lower by a factor of about 50 when the ribonucleoside is fed. The physiological effects of zeatin and abscisic acid on the germination of ash embryos are antagonistic. There is, however, no evidence that abscisic acid has a significant effect on 8-¹⁴C-zeatin uptake or conversions.     

Characterization of Cadmium Binding, Uptake, and Translocation in Intact Seedlings of Bread and Durum Wheat Cultivars

High Cd content in durum wheat (Triticum turgidum L. var durum) grain grown in the United States and Canada presents potential health and economic problems for consumers and growers. In an effort to understand the biological processes that result in excess Cd accumulation, root Cd uptake and xylem translocation to shoots in seedlings of bread wheat (Triticum aestivum L.) and durum wheat cultivars were studied. Whole-plant Cd accumulation was somewhat greater in the bread wheat cultivar, but this was probably because of increased apoplastic Cd binding. Concentration-dependent ¹⁰⁹Cd²⁺-influx kinetics in both cultivars were characterized by smooth, nonsaturating curves that could be dissected into linear and saturable components. The saturable component likely represented carrier-mediated Cd influx across root-cell plasma membranes (Michaelis constant, 20–40 nm; maximum initial velocity, 26–29 nmol g⁻¹ fresh weight h⁻¹), whereas linear Cd uptake represented cell wall binding of ¹⁰⁹Cd. Cd translocation to shoots was greater in the bread wheat cultivar than in the durum cultivar because a larger proportion of root-absorbed Cd moved to shoots. Our results indicate that excess Cd accumulation in durum wheat grain is not correlated with seedling-root influx rates or root-to-shoot translocation, but may be related to phloem-mediated Cd transport to the grain.     

P-NMR Spectroscopy of Roots of Intact Corn Seedlings

The application of ³¹P nuclear magnetic resonance spectroscopy to the study of metabolism in roots of intact corn seedlings is described. ³¹P-NMR spectra of developmentally distinct parts of primary roots of whole seedlings are presented. The spectra are of quality comparable to those of excised pieces of plant tissue.     

Translocation and Metabolism of Leaf Applied Hexachlorophene in Peanuts

Translocation, distribution, metabolism and photolysis of hexachlorophene (HCP) were investigated in peanut plants (Arachis hypogea L., Spanish type) grown under standardized conditions and treated with ^l4C-ring-labeled HCP. Treatment time ranged from 0–114 days. Autoradiographic analyses were performed on all plants. Selected plant tissues were extracted and chromatographed, using both thin layer and gas liquid chromatography. No translocation of HCP was detected in the plant tissue. No HCP metabolites were found. Some HCP was lost from the leaves and inert controls at a specific rate per unit time. The rates were slightly different, being slower on the leaves than on the controls. At the end of the 114-day treatment and based on regression analysis of thin layer chromatographic plates, an average of 68% of the applied HCP remained unaltered on the treated plants and an average of 77% remained on the controls. This indicated that, respectively, 32 and 23% of the original HCP had been altered. This 9% difference was statistically significant. Upon further analyses of the above data, using gas chromatographic methods, as many as 14 peaks were found in the treated samples and the controls, including some parent material. Ultraviolet photolysis seems to be the mechanism responsible for alteration of the HCP on the treated plants and controls. Two photolysis products have been identified by gas liquid chromatography-mass spectral analysis. Twelve other electrophylic compounds have been found in various treated plant or control extracts. Further analyses will be necessary to verify the identification and quantification of the other degradation products.     

Kinetics of Manganese Uptake by Intact Citrus Seedlings

Uptake of manganese by intact citrus seedlings can be represented by three phases of a single, multiphasic isotherm in the range 10^?8M–2× 10^?4M. The phases are separated by marked jumps and the kinetic constants increase upon transition to higher phases.